Place: International Conference on Solid-Solid Phase Transformations, PTM 2010, Avignon, France
Taichi Abe, Research Center for Structural Materials, National Institute for Materials Science, Tsukuba, Ibaraki, Japan
Nils Warnken, Department of Metallurgy and Materials, University of Birmingham, Birmingham, United Kingdom
It is well recognized that refractory elements as Re are very effective in improving the mechanic properties of Ni-base superalloys. Moreover, Re is an important element in alloys which are used as coating materials for these alloys in order to prevent degradation processes. Under service conditions at high temperatures, however, precipitation of TCP (Topologically Close-Packed) phases, both in the superalloy and in the interface between them and the coating layer, can significantly decrease material properties, especially creep and oxidation resistance. Microstructure simulations can help to understand the precipitation process of TCP phases and thus overcome the present drawbacks in Re-base coatings and Ni-base superalloys.
In this work, binary and ternary phases of the Cr-Ni-Re system were carefully reexamined and revisited using the CALPHAD approach and the ThermoCalc program. The five crystallographic sublattices of the sigma phase were modelled in the framework of the CEF (Compound Energy Formalism) for the sigma phase, both in the binaries and ternary systems. DFT calculations and the VASP code have been used to determine the enthalpies of formation at 0 K of all possible configurations when Cr, Ni and Re occupy different Wyckoff positions in sigma phase. From DFT results site occupancies have been calculated and the enthalpies of formation were used for the end-members energies in the five-sublattice model. The assessed thermodynamic Gibbs energy has thus been used for phase field simulations of nucleation and growth of sigma phase. Results of calculations were compared with the available experimental data.